Polarizing beam splitter

ABSTRACT

The present invention relates to a polarizing beam splitter comprised of a film made of a high-refractivity material and a film made of a low-refractivity material alternately stacked on a substrate. When an incidence angle with respect to a film surface in a particular wavelength is set to be θ (°), conditional expressions: 0.99≦Rs(45)/Rs(θ)≦1.04, 0.96≦Tp(45)/Tp(θ)≦1.05 {Rs(θ): reflectivity of polarized light s at an incidence angle of θ, Tp(θ): transmissivity of polarized light p at an incidence angle of θ, Rs(45): reflectivity of polarized light s at the incidence angle of 45°, Tp(45): transmissivity of polarized light p at the incidence angle of 45°} are satisfied in an entire range of 40≦θ≦50 (°). According to the configuration, the transmissivity of the polarized light p can be prevented from decreasing while maintaining the reflectivity of the polarized light s at nearly 100% with respect to even a broad angle region, where a divergence angle of an incident light is ±5° or above.

The present application claims priority to Japanese Patent Application No. 2003-284722 filed Aug. 1, 2003, the entire content of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polarizing beam splitter, more particularly to a polarizing beam splitter capable of a high-level performance suitable for, for example, a light pickup optical system using blue laser, a projection optical system, or the like, and a polarizing, beam splitter film as a component thereof.

2. Description of the Related Art

In a light pickup optical system for a blue region requires a polarizing beam splitter having such polarization separation characteristics that a transmissivity of a polarized light p and a reflectivity of a polarized light s in a wavelength region of around 405 nm both mark nearly 100%. Currently, however, a divergence angle of blue laser is significantly large. Therefore, when an ordinary polarizing beam splitter film is used, the transmissivity of the polarized light p is largely decreased due to a variation of an incidence angle.

Japanese Unexamined Patent Publication Nos. 08-146218 and 09-184916 disclose, in order to solve such a problem, polarizing beam splitters having predetermined polarization separation characteristics with respect to even a broad angle region, where a divergence angle of an incident light is ±5° or above. For example, a polarizing beam splitter film disclosed in No. 09-184916 comprises, when a central wavelength of a split light is λ, an optical film thickness of a film made of a high-refractivity material is H, and an optical film thickness of a film made of a low-refractivity material is L, a first stack constituting 0.8×λ/4≦H≦1×λ/4 and 0.7×λ/4≦L≦1×λ/4, and a second stack constituting 1.3×λ/4≦H≦1.5×λ/4 and 1.2×λ/4≦L≦1.5×λ/4.

However, in the case of polarizing beam splitter films disclosed in No. 08-146218 and No. 9-184916, an angle dependency of the polarized light s can only be controlled at approximately 20% in terms of the transmissivity, which shows insufficient polarization separation characteristics thereof. Therefore, when the polarizing beam splitter film is used in the light pickup optical system using the blue laser, projection optical system, or the like, such a problem as an decreased quantity of light is generated.

SUMMARY OF THE INVENTION

A main object of the present invention is to provide a polarizing beam splitter and a polarizing beam splitter film capable of preventing a transmissivity of a polarized light p from decreasing while maintaining a reflectivity of a polarized light s at nearly 100% with respect to even a broad angle region, where a divergence angle of an incident light is ±5° or above.

In order to achieve the foregoing object and other objects as well, in an stage of the present invention, a polarizing beam splitter having a multilayer structure, wherein a film made of a high-refractivity material and a film made of a low-refractivity are alternately stacked on a substrate, satisfies the following conditional expressions 1A and 1B in an entire range of 40≦θ≦50 (°) when an incidence angle with respect to a film surface in a particular wavelength is set to be θ (°). 0.99≦Rs(45)/Rs(θ)≦1.04  1A 0.96≦Tp(45)/Tp(θ)≦1.05  1B

-   -   providing that,

Rs(θ): reflectivity of polarized light s at an incidence angle of θ

Tp(θ): transmissivity of polarized light p at an incidence angle of θ

Rs(45): reflectivity of polarized light s at the incidence angle of 45°.

Tp(45): transmissivity of polarized light p at the incidence angle of 45°.

The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a spectral characteristic with respect to an incidence angle according to an embodiment 1 of the present invention;

FIG. 2 is a graph showing a spectral characteristic with respect to a wavelength according to the embodiment 1;

FIG. 3 is a graph showing a spectral characteristic with respect to an incidence angle according to an embodiment 2 of the present invention; and

FIG. 4 is a graph showing a spectral characteristic with respect to a wavelength according to the embodiment 2.

In the following description, like components are designated by like reference numbers throughout the several drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a polarizing beam splitter and a polarizing beam splitter film executed by the present invention are described referring to the drawings. Tables 1 and 2 respectively show multilayer films according to embodiments 1 and 2 (QWOT=4·n·d/λ, d: physical film thickness, n: refractivity, λ: wavelength) as the polarizing beam splitter film according to the present invention. In the embodiment 1, a film made of a high-refractivity material: TiO₂ (titanium oxide) and a film made of a low-refractivity material: SiO₂ (silicon oxide) are alternately stacked to thereby form 33 layers on a glass substrate having the refractivity of 1.64 in the order of layer numbers. In the embodiment 2, a film made of a blended material TX including a high-refractivity material: TiO₂ (titanium oxide) and a film made of a low-refractivity material: MgF₂ (magnesium fluoride) or SiO₂ (silicon oxide) are alternately stacked to thereby form 25 layers on a glass substrate having the reflectivity of 1.64 in the order of the layer numbers.

In the case of the polarizing beam splitter film comprised of the film made of the high-refractivity material and the film made of the low-refractivity material alternately stacked as in the embodiments 1 and 2, it is preferable to satisfy the following conditional expressions 1A and 1B in an entire range of 40≦θ≦50 (°) when an incidence angle with respect to a film surface in a particular wavelength is set to be θ (°). 0.99≦Rs(45)/Rs(θ)≦1.04  1A 0.96≦Tp(45)/Tp(θ)≦1.05  1B

-   -   providing that,

Rs(θ): reflectivity of polarized light s at an incidence angle of θ

Tp(θ): transmissivity of polarized light p at an incidence angle of θ

Rs(45): reflectivity of polarized light s at the incidence angle of 45°

Tp(45): transmissivity of polarized light p at the incidence angle of 45°

Tables 3 and 4 show data according to respective embodiments in the case of wavelength λ=405 nm and 40≦θ≦50 (°): reflectivity Rs (θ) of polarized light s and transmissivity Tp (θ) of polarized light p, and data corresponding to parameters regulated in the conditional expressions 1A and 1B. As learnt from the tables 3 and 4, the embodiments 1 and 2 satisfy the conditional expressions 1A and 1B in the entire range of 40≦θ≦50 (°). Thus, when the conditional expressions 1A and 1B are satisfied in the entire range of 40≦θ≦50 (°), the transmissivity of the polarized light p can be prevented from decreasing while maintaining the reflectivity of the polarized light s at nearly 100% with respect to even a broad angle region, where a divergence angle of an incident light is ±5° or above.

As described, in order to satisfy the conditional expressions 1A and 1B in the entire range of 40≦θ≦50 (°), it is preferable to have at least a stack comprised of a film made of a high-refractivity material satisfying the following conditional expression 2A and a film made of a low-refractivity material satisfying the following conditional expression 2B alternately stacked in two or more repetitive cycles, providing that a pair of a layer of the film made of the high-refractivity material and a layer of the film made of the low-refractivity material constitutes a cycle. 0.7×λ/4≦H≦1×λ/4  2A 1×λ/4≦L≦2×λ/4  2B

providing that,

H: optical film thickness of film made of high-refractivity material

L: optical film thickness of film made of low-refractivity material

λ: central wavelength of split light

In the embodiment 1, a multilayer structure comprised of layers from a 16th layer through a 27th layer (layer No. 16-27) constitutes a stack. The stack is comprised of a film made of TiO₂ satisfying the conditional expression 2A and a film made of SiO₂ satisfying the conditional expression 2B alternately stacked in six repetitive cycles. In the embodiment 2, a multilayer structure comprised of layers from a 3rd layer through a 14th layer (layer No. 3-14) constitutes a stack. The stack is comprised of a film made of TX satisfying the conditional expression 2A and a film made of SiO₂ or MgF₂ satisfying the conditional expression 2B alternately stacked in six repetitive cycles. The adoption of such a configuration having a discriminative stack is capable of arranging an angle dependency of the polarized light s to invariably lead to the transmissivity of approximately 0%. Therefore, in an optical system, in which the polarized lights p and s are required to be sufficiently polarized and separated despite the divergence angle of the incident light being large (for example, light pickup optical system using blue laser, projection optical system, or the like), a quantity of light and optical performance can be dramatically improved.

FIGS. 1 and 2 show the polarization separation characteristics according to the embodiment 1 by means of the transmissivity T (%). FIG. 1 shows Tp (θ) , which is the transmissivity of the polarized light p, and Ts (θ), which is the transmissivity of the polarized light s in the case of the wavelength λ=405 nm and the incidence angle with respect to the film surface θ=40-50°. FIG. 2 shows Tp (θ), which is the transmissivity of the polarized light p, and Ts (θ), which is the transmissivity of the polarized light s in the case of the wavelength λ=300-500 nm and the incidence angle with respect to the film surface θ=40°, 45°, and 50°. FIGS. 3 and 4 show the polarization separation characteristics according to the embodiment 2 by means of the transmissivity T (%). FIG. 3 shows Tp (θ), which is the transmissivity of the polarized light p, and Ts (θ), which is the transmissivity of the polarized light s in the case of the wavelength λ=405 nm and the incidence angle with respect to the film surface θ=40-50°. FIG. 4 shows Tp (θ), which is the transmissivity of the polarized light p, and Ts (θ), which is the transmissivity of the polarized light s in the case of the wavelength λ=300-500 nm and the incidence angle with respect to the film surface θ=40°, 45°, and 50°. As learnt from FIGS. 1 through 4, any of the embodiments comprises the polarization separation characteristics suitable for the polarizing beam splitter for blue laser. TABLE 1 Example 1 Physical film Layer thickness QWOT No. Material d (nm) (4 · n · d/λ) 1 SiO₂ 131.17 1.900 2 TiO₂ 77.41 1.924 3 SiO₂ 122.38 1.773 4 TiO₂ 58.06 1.443 5 SiO₂ 129.01 1.869 6 TiO₂ 66.92 1.663 7 SiO₂ 110.94 1.607 8 TiO₂ 55.71 1.385 9 SiO₂ 110.13 1.595 10 TiO₂ 65.6 1.631 11 SiO₂ 140.19 2.031 12 TiO₂ 62.1 1.543 13 SiO₂ 95.96 1.390 14 TiO₂ 34.82 0.865 15 SiO₂ 65.77 0.953 16 TiO₂ 32.51 0.808 17 SiO₂ 78.93 1.143 18 TiO₂ 39.28 0.976 19 SiO₂ 87.41 1.266 20 TiO₂ 37.04 0.921 21 SiO₂ 82.48 1.195 22 TiO₂ 32.38 0.805 23 SiO₂ 84.14 1.219 24 TiO₂ 34.71 0.863 25 SiO₂ 88.87 1.287 26 TiO₂ 35.27 0.877 27 SiO₂ 79.75 1.155 28 TiO₂ 31.22 0.776 29 SiO₂ 280.41 4.062 30 TiO₂ 31.47 0.782 31 SiO₂ 89.89 1.302 32 TiO₂ 95.96 2.385 33 SiO₂ 69.04 1.000

TABLE 2 Example 2 Physical film Layer thickness QWOT No. Material d (nm) (4 · n · d/λ) 1 TX 169.7 3.555 2 MgF₂ 263.76 3.608 3 TX 38.96 0.816 4 SiO₂ 94.45 1.368 5 TX 40.59 0.851 6 MgF₂ 106.31 1.454 7 TX 42.69 0.894 8 SiO₂ 97.01 1.405 9 TX 39.52 0.828 10 SiO₂ 87.8 1.272 11 TX 39.26 0.823 12 MgF₂ 103.89 1.421 13 TX 39.6 0.830 14 SiO₂ 107.87 1.563 15 TX 60.45 1.266 16 MgF₂ 179.6 2.457 17 TX 20.82 0.436 18 SiO₂ 195.14 2.827 19 TX 18.29 0.383 20 MgF₂ 177.36 2.426 21 TX 58.69 1.230 22 SiO₂ 122.25 1.771 23 TX 53.07 1.112 24 SiO₂ 364.74 5.283 25 TX 74.08 1.552

TABLE 3 Example 1 θ (°) Rs (θ) Tp (θ) (1A): Rs(45)/Rs(θ) (1B): Tp(45)/Tp(θ) 40 99.954 98.921 1.000 1.008 41 99.974 99.475 1.000 1.002 42 99.989 99.928 1.000 0.997 43 99.995 99.397 1.000 1.003 44 99.997 99.102 1.000 1.006 45 99.998 99.668 1.000 1.000 46 99.999 99.634 1.000 1.000 47 100.000 99.337 1.000 1.003 48 100.000 99.876 1.000 0.998 49 100.000 99.306 1.000 1.004 50 100.000 98.746 1.000 1.009

TABLE 4 Example 1 θ (°) Rs (θ) Tp (θ) (1A): Rs(45)/Rs(θ) (1B): Tp(45)/Tp(θ) 40 99.954 98.921 1.000 1.008 41 99.974 99.475 1.000 1.002 42 99.989 99.928 1.000 0.997 43 99.995 99.397 1.000 1.003 44 99.997 99.102 1.000 1.006 45 99.998 99.668 1.000 1.000 46 99.999 99.634 1.000 1.000 47 100.000 99.337 1.000 1.003 48 100.000 99.876 1.000 0.998 49 100.000 99.306 1.000 1.004 50 100.000 98.746 1.000 1.009

Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein. 

1. A polarizing beam splitter having a multilayer structure comprised of a film made of a high-refractivity material and a film made of a low-refractivity material alternately stacked on a substrate, wherein the following conditional expressions 1A and 1B are satisfied in an entire range of 40≦0≦50 (°) when an incidence angle with respect to a film surface in a particular wavelength is set to be θ (°) 0.99≦Rs(45)/Rs(θ)≦1.04  1A 0.96≦Tp(45)/Tp(θ)≦1.05  1Bproviding that, Rs(θ): reflectivity of polarized light s at an incidence angle of θ Tp(θ): transmissivity of polarized light p at an incidence angle of θ Rs(45): reflectivity of polarized light s at the incidence angle of 45° Tp(45): transmissivity of polarized light p at the incidence angle of 45°
 2. A polarizing beam splitter as claimed in claim 1, the polarizing beam splitter having at least a stack comprised of a film made of a high-refractivity material satisfying the following conditional expression 2A and a film made of a low-refractivity material satisfying the following conditional expression 2B alternately stacked in at least two repetitive cycles. 0.7×λ/4≦H≦1×λ/4  2A 1×λ/4≦L≦2×λ/4  2Bproviding that, H: optical film thickness of film made of high-refractivity material L: optical film thickness of film made of low-refractivity material λ: central wavelength of split light
 3. A polarizing beam splitter film having a multilayer structure comprised of a film made of a high-refractivity material and a film made of a low-refractivity material alternately stacked on a substrate, wherein the following conditional expressions 1A and 1B are satisfied in an entire range of 40≦θ≦50 (°) when an incidence angle with respect to a film surface in a particular wavelength is set to be θ (°). 0.99≦Rs(45)/Rs(θ)≦1.04  1A 0.96 ≦Tp(45)/Tp(θ)≦1.05  1Bproviding that, Rs(θ): reflectivity of polarized light s at an incidence angle of θ Tp(θ): transmissivity of polarized light p at an incidence angle of θ Rs(45): reflectivity of polarized light s at the incidence angle of 45° Tp(45): transmissivity of polarized light p at the incidence angle of 45°
 4. A polarizing beam splitter as claimed in claim 3, the polarizing beam splitter having at least a stack comprised of a film made of a high-refractivity material satisfying the following conditional expression 2A and a film made of a low-refractivity material satisfying the following conditional expression 2B alternately stacked in at least two repetitive cycles. 0.7×λ/4≦H≦1×λ/4  2A 1×λ/4≦L≦2×λ/4  2B providing that, H: optical film thickness of film made of high-refractivity material L: optical film thickness of film made of low-refractivity material λ: central wavelength of split light 